Down to the Wire

English

 

Platinum wires attached to a single organic molecule (center) can be elongated into a chain a single atom thick

 

The never-ending trend of miniaturization in electronics hits the wall when things get down to the nanometer scale. At this point, it is not enough to make the same device but smaller; new technology is needed. That is why scientists investigate the use of single molecules for electronics. They already know how to get such molecules to conduct electricity, and even how to manipulate the structure of these molecules so as to control the electric current moving through them (in much the same way as a drug molecule’s structure will determine its actions in the body). For example, molecules have been designed to act as switches or one-way valves that regulate the direction of flow of current.

 

However, to use these molecules in electronic circuits, they must be able to connect with metal wires. Until now, such molecules have been held in place between electric wires before use, but even the smallest wires are several orders of magnitude larger than the molecules. Dr. Oren Tal and research students Tamar Yelin, Ran Vardimon and Natalia Kuritz of the Chemical Physics Department recently took a significant step toward bringing the wires into line with the molecules. They managed to connect a single organic molecule to the thinnest electric wire possible: a single-file string of platinum atoms.
 

 

Dr. Oren Tal

 

The scientists first trapped a single molecule between two much thicker platinum wires, and then, immediately afterward, they moved the wires away from each other until the platinum atoms in them were pulled into a chain of atoms that was connected to the molecule on one end and a standard metal wire on the other.

 
The research showed that the electrical conductance of the organic molecule-platinum wire setup was not significantly suppressed by elongating the chains with additional atoms. This implies that such systems might be useful for transferring electronic signals over distances without a significant reduction in intensity. The scientists also experimented with different molecules and molecular structures; they found that different molecules can be wired up in this way. Thus the method could potentially have a wide variety of applications.


Also participating in this research – and helping the group decipher the properties of the new system and the chemical nature of the connection between the atomic strings and the molecule – were the research groups of Prof. Leeor Kronik of the Weizmann Institute’s Materials and Interfaces Department and Prof. Ferdinand Evers of the Karlsruhe Technical Institute in Germany.


Tal’s group and others have already begun to investigate the behavior of electric current when it passes through single molecules connected to the ultra-thin platinum wires. Among other things, the contact points between two very different nanostructures – organic molecules and strings of metallic atoms – may provide new and unique ways of controlling electric current on the sub-nanometer scale.
 

Prof. Leeor Kronik's research is supported by the Wolfson Family Charitable Trust; the Carolito Stiftung; the European Research Council; the Leona M. and Harry B. Helmsley Charitable Trust; Antonio and Noga Villalon, Winnetka, IL; and the Philip M. Klutznick Fund for Research.

 

Dr. Oren Tal's research is supported by the Carolito Stiftung. Dr. Tal is the incumbent of the Alvin and Gertrude Levine Career Development Chair.
 

 
Platinum wires attached to a single organic molecule (center) can be elongated into a chain a single atom thick
Chemistry
English
  • Chemical Physics
  • Leeor Kronik
  • Materials and Interfaces
  • Nanoelectronics
  • Oren Tal
  • Organic molecule

Guided Growth of Nanowires Leads to Self-Integrated Circuits

English

 

SEM image of a logic circuit based on 14 nanowires
 
 
Researchers working with tiny components in nanoelectronics face a challenge similar to that of parents of small children: teaching them to manage on their own. The nano-components are so small that arranging them with external tools is impossible. The only solution is to create conditions in which they can be “trusted” to assemble themselves.

Much effort has gone into facilitating the self-assembly of semiconductors, the basic building blocks of electronics, but until recently, success has been limited. Scientists had developed methods for growing semiconductor nanowires vertically on a surface, but the resultant structures were short and disorganized. After growing, such nanowires need to be “harvested” and aligned horizontally; since such placement is random, scientists need to determine their location and only then integrate them into electric circuits.

A team led by Prof. Ernesto Joselevich of the Weizmann Institute’s Materials and Interfaces Department has managed to overcome these limitations. For the first time, the scientists have created self-integrating nanowires whose position, length and direction can be fully controlled.

The achievement, reported this week in the Proceedings of the National Academy of Sciences (PNAS), USA, was based on a method developed by Joselevich two years ago for growing nanowires horizontally in an orderly manner. In the present study – conducted by Joselevich with Dr. Mark Schvartzman and David Tsivion of his lab, and Olga Raslin and Dr. Diana Mahalu of the Physics of Condensed Matter Department – the scientists went further, creating self-integrated electronic circuits from the nanowires.

First, the scientists prepared a surface with tiny, atom-sized grooves and then added to the middle of the grooves catalyst particles that served as nuclei for the growth of nanowires. This setup defined the position, length and direction of the nanowires. They then succeeded in creating a transistor from each nanowire on the surface, producing hundreds of such transistors simultaneously. The nanowires were also used to create a more complex electronic component – a functioning logic circuit called an Address Decoder, an essential constituent of computers. These ideas and findings have earned Joselevich a prestigious European Research Council Advanced Grant.

“Our method makes it possible, for the first time, to determine the arrangement of the nanowires in advance to suit the desired electronic circuit,” Joselevich explains. The ability to efficiently produce circuits from self-integrating semiconductors opens the door to a variety of technological applications, including the development of improved LED devices, lasers and solar cells.
 
Prof. Ernesto Joselevich's research is supported by the Carolito Stiftung and the European Research Council.
 
 
SEM image of a logic circuit based on 14 nanowires
Chemistry
English
  • Condensed matter Physics
  • Ernesto Joselevich
  • Logic gate
  • Materials and Interfaces
  • Nanoelectronics
  • Nanowire

Getting Nanowires into the Groove

English

Mistakes are the portals of discovery.
     - James Joyce
 

Top view of the nanowires by scanning electron microscopy (SEM)
Growing up is not easy. Take nanowires: With no support or guidance, they become unruly, making it difficult to harness their full potential. Prof. Ernesto Joselevich of the Weizmann Institute’s Chemistry Faculty has found a way to grow nanowires out – not up –  providing the support and guidance they need to become long, orderly aligned structures. Since semiconductors with controlled structures are at the core of the most advanced technologies, this new research will hopefully enable the production of semiconductor nanostructures with enhanced electronic and optical properties with a wide range of applications, among them transistors, LEDs, lasers, information storage media computers and photovoltaics.
 
Most nanowires actually start out with a successful “upbringing”: In the so-called vapor-liquid-solid (VLS) technique, vaporized raw semiconductor materials are dissolved in a tiny metal droplet placed on a surface. The semiconducting nanowire grows upward – a bit like a stalagmite – as the material crystallizes and more is deposited. Such wires are defect-free and exhibit excellent optical and electronic properties, as their lack of contact with the surface allows them to grow up in a stress-free environment. The problems show up later, when it is time to harvest the nanowires and assemble them into structured arrays. Then, the fragile nanowires often break into little pieces, get entangled and resist alignment – limiting their use in many applications. But horizontal growth, which might yield stronger nanowires, also has its shortcomings: The underlying surface usually introduces stress, causing defects and leading to subpar optical and electronic properties in the semiconductor material.
 
Joselevich and his team’s first hint that these shortcomings might be surmountable came in earlier research, when they discovered that growing carbon nanotubes on sapphire that had mistakenly been cut a few degrees off from the main crystal plane mysteriously resulted in the growth of well-aligned, millimeter-long nanotubes. Upon closer inspection, they found that the uneven cut had created “steps” of atomic dimensions between the crystal’s planes, guiding the growth of the nanotubes into orderly, well-aligned arrays.
 
Illustration of nanowires growing along nanogrooves
 
 
 
In their new research, Joselevich, together with Ph.D. student David Tsivion and postdoctoral fellow Dr. Mark Schvartzman of the Materials and Interfaces Department, took this technique a step further and for the first time used sapphire steps to try to control the horizontal growth of nanowires. This time, they deliberately cut the sapphire along different planes of the crystal, resulting in a variety of surface patterns: smooth, step-like and accordion-like (V-shaped) grooves. They then used the VLS method to grow nanowires of gallium nitride (GaN) – a popular semiconducting material that is the basis of the blue LEDs used in outdoor TV screens and the violet lasers used in Blu-ray discs.
 
Their results, which appeared in Science, show that, unlike smooth surfaces, steps and grooves have a strong guiding effect: The nanowires grow horizontally along the edges of and within the grooves into well-aligned, millimeter-long structures. The scientists found that they could even control the orientation of the atoms within the nanowire by altering the direction of the sapphire cut. This orientation is known to affect the nanowires’ properties, and it could be relevant to photonic, optoelectronic and radio frequency applications.
 
The scientists were pleased to find that their nanowires’ electrical and optical properties were just as good as – if not better than – those of vertically-grown nanowires. Joselevich: “This was very surprising, as we expected to see at least some degradation in quality due to contact with the surface.”
 
Although it is still not clear exactly how the VLS method produces horizontal nanowires, Joselevich and his team have managed to combine, in a single step, the synthesis and assembly of well-structured nanowires with unique properties suitable for a wide range of applications, simply by getting them “into the groove.”
 
 
 
 
 
(l-r) Dr. Mark Schvartzman, Prof. Ernesto Joselevich and David Tsivion. New nanowire horizons
 
 

 

 

Edible nanowires


Having a paper accepted to Science is cause for celebration. To let his students in on the good news, Joselevich summoned them to an “urgent meeting,” where he surprised them with cake and champagne. The orange sponge cake was baked by his wife, but Joselevich supplied the decoration: He sliced grooves into the cake’s surface and frosted it with white chocolate, topping the grooves with “nanowires” in yellow icing and silver sugared almonds representing the wires’ nanoparticle catalyst.  
 
Prof.  Ernesto Joselevich’s research is supported by the Carolito Stiftung.


 

 

 
 
Illustration of nanowires growing along nanogrooves
Chemistry
English
  • Ernesto Joselevich
  • Materials and Interfaces
  • Nanoscience
  • Nanowire

Nanowires Get into the Groove

English

22-08-2011
 

Illustration of nanowires growing along nanogrooves
 
Growing up is not easy, especially for tiny nanowires: With no support or guidance, nanowires become unruly, making it difficult to harness their full potential as effective semiconductors. Prof. Ernesto Joselevich of the Weizmann Institute’s Chemistry Faculty has found a way to grow semiconductor nanowires out, not up, on a surface, providing, for the first time, the much-needed guidance to produce relatively long, orderly, aligned structures. Since semiconductors with controlled structures are at the core of the most advanced technologies, this new research will hopefully enable the production of semiconductor nanostructures with enhanced electronic and optical properties, suitable for a wide range of applications including LEDs, lasers, information storage media, transistors, computers, photovoltaics and more.

Joselevich, Ph.D. student David Tsivion and postdoctoral fellow Mark Schvartzman of the Materials and Interfaces Department grew nanowires made of gallium nitride (GaN) using a method that usually produces vertical nanowires with excellent optical and electronic properties. These vertical wires only become unruly once they are harvested and assembled into arrays. To bypass this problem, the scientists used sapphire as a base on which to grow the nanowires. But rather than growing them on a smooth surface, they deliberately cut the sapphire along different planes of the crystal, resulting in various surface patterns including “steps” of nanometer dimensions between the different planes, as well as accordion-like, V-shaped grooves.
 

 

Top view of the nanowires by scanning electron microscopy (SEM)
 
Their results, recently published in Science, show that surface steps and grooves have a strong guiding effect, coaxing the nanowires to grow horizontally along their edges or within the grooves and producing well-aligned, millimeter-long nanowire arrays. In contrast, current methods of assembling nanowires horizontally on smooth surfaces result in disorderly nanowires only micrometers in length with subpar properties.

Joselevich: “It was surprising to discover that the optical and electronic properties of our nanowires were just as good – if not better – than those grown vertically, because growing semiconductors on a surface usually introduces defects that degrade their quality.”  

Although it is still not fully clear how a method that normally produces vertical nanowires works to create horizontal growth in the new study, Joselevich and his team have managed to combine, in a single step, the synthesis and assembly of well-structured nanowires with unique properties suitable for a wide range of applications, by simply getting them “into the groove.” 
 
 
 
Illustration of nanowires growing along nanogrooves
Chemistry
English
  • Ernesto Joselevich
  • Materials and Interfaces
  • Nanoscience
  • Nanowire

Spinning off DNA

English
 
 
(l-r) Tal Markus, and Profs. Zeev Vager and Ron Naaman. Putting a spin on it
 
Biological molecules and quantum systems are not just apples and oranges – they’re more like apple trees and cans of frozen orange juice. The two exist in completely different conditions, on completely different scales. Yet research by Prof. Ron Naaman of the Chemical Physics Department (Faculty of Chemistry), conducted together with scientists at the Weizmann Institute and in Germany, definitively shows that a biological molecule – DNA – can discern between the quantum states known as spin in such subatomic particles as electrons.

Biological molecules are chiral: They exist in either “left-” or “right-handed” forms that can’t be superimposed on one another. Double-stranded DNA molecules are doubly chiral – both in the arrangement of the individual strands and in the direction of their helical twists. Naaman, together with Prof. Zeev Vager of the Particle Physics and Astrophysics Department, research student Tal Markus, and Prof. Helmut Zacharias and his research team at the University of Münster, Germany, thought that this chirality might imbue DNA with spin-selective properties. Their findings appeared in Science.

The researchers exposed DNA to groups of electrons with both directions of spin. Indeed, the team’s results surpassed expectations: The biological molecules reacted strongly with the electrons carrying one of those spins, and hardly at all with the others. Their findings imply that the chiral nature of the DNA molecule somehow “sets the preference” for the spin of electrons moving through it.

The team’s findings could have relevance for both biomedical research and the field of spintronics. For instance, if further studies bear out the finding that DNA sustains damage only from spins pointing in one direction, then medical radiation exposure might be reduced and devices designed accordingly. On the other hand, DNA and other biological molecules could become a central feature of new types of spintronic devices that will work on particle spin rather than electric charge, as they do today.
 
Prof. Ron Naaman heads the Nancy and Stephen Grand Research Center for Sensors and Security; his research is also supported by the estate of Theodore Rifkin; and Rachel Schwartz, Canada. Prof. Naaman is the incumbent of the Aryeh and Mintzi Katzman Professorial Chair.
 
 
 
(l-r) Tal Markus, and Profs. Zeev Vager and Ron Naaman. Putting a spin on it
Chemistry
English
  • Chemical Physics
  • Chiral
  • DNA
  • Particle Physics
  • Ron Naaman
  • Spin›Spintronics
  • Zeev Vager

Shine a Light on the Nanoworld

English

(l-r) Drs. Eugene Katz, Ronit Popovitz-Biro and Maya Bar-Sadan, Profs. Daniel Feuermann, Reshef Tenne, Jeffrey Gordon and Moshe Levy, and Dr. Ana Albu-Yaron. Nanoparticles

 
   
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Sunlight – valued as a nearly infinite source of energy – can also provide an unusual glimpse into the nanoworld. In a new study, led by researchers from the Weizmann Institute and Ben-Gurion University of the Negev, highly concentrated solar radiation helped reveal the shapes assumed by certain inorganic nanoparticles. This research, conducted in collaboration with German scientists, sheds new light, as it were, on the behavior of particles on the nanoscale and could lead to advanced uses for various nanomaterials.
 
Nearly two decades ago, the Weizmann Institute’s Prof. Reshef Tenne and his colleagues in the Chemistry Faculty were the first to discover that inorganic materials could form hollow, cage-like nanostructures. Until then, only carbon molecules were known to form hollow spheres, which came to be called fullerenes due to their likeness to the geodesic domes built by the architect Buckminster Fuller. Tenne’s discovery opened up a new field of research into the inorganic fullerene-like particles, creating exciting applications, including the manufacture of superior solid lubricants, and posing a host of new questions, particularly those concerning the connection between the particles’ superior properties and their structure and shape.
 
The “true” inorganic fullerene – the smallest, most stable cage-like particle of inorganic material – is shaped like an octahedron resembling an eight-sided die. Such tiny octahedra have already been produced at the Weizmann Institute and elsewhere. But larger inorganic fullerene-like structures have also been produced, these being multi-layer spheres. At what point, exactly, does the octahedron become spherical in shape?
 
 
An intermediate molybdenum disulfide nanoparticle has an octahedral center and a spherical outer shape
 
 
This is a crucial question – in part because the two shapes appeared to endow the nanoparticles with different properties – and it has been answered by the new study, published recently in the international edition of Angewandte Chemie. The international team worked with molybdenum disulfide (MoS2) nanoparticles just a few millionths of a meter across.
 
Their first goal was to create an atomic vapor of molybdenum disulfide. Laser light, used in previous nanoscale studies with this material, was found to produce only small octahedra made up of 20,000 atoms. To generate larger octahedra, Ben-Gurion University researchers headed by Prof. Jeffrey Gordon built an innovative table-top solar concentrator consisting of an elaborate system of mirrors that created an ultra-intense solar beam focused to a magnitude of about 15,000 suns. Inside a quartz capsule, molybdenum disulfide heated to 2,500oC was vaporized into a hot cloud of individual atoms. Since the concentrated solar beam is appreciably wider than the beams of typical pulsed lasers, the evaporated atoms could, upon cooling, form much larger clusters than those obtained with lasers.
 
Lead author of the study Dr. Ana Albu-Yaron of the Weizmann Institute and her colleagues used a series of electron microscopes – including the most advanced one in Dr. Lothar Houben’s laboratory at the Jülich Research Center in Germany – to view the architecture of these nanoparticles. The picture that emerged provided the first support for certain theoretical predictions in this area made by Prof. Gotthard Seifert’s group at the Technical University of Dresden, which turned out to be impressively accurate.
 
In addition to the distinct eight-sided and spherical shells, the team observed hybrid nanoparticles of molybdenum disulfide assuming an intermediate “transition” shape, and the study revealed precisely at which size each shape occurs. The smallest molecules, made up of no more than 100,000 atoms, were hollow octahedra. Larger particles, comprising about 500,000 atoms, had an intermediate structure: octahedral layers at the core, surrounded by multiple onion-like spherical shells.
 
Beyond addressing fundamental questions in materials science, these results can be of practical significance. Molybdenum disulfide is used as a catalyst for removing sulfur from fossil fuels to prevent acid rain. In the form of nanoparticles, the catalyst could be much more effective: Thanks to their voluminous, three-dimensional structure, such particles are likely to be more accessible to interaction with the sulfur, speeding up the removal process. The researchers plan to explore the potential of their solar-generated nanoparticles as catalysts once they manage to produce them in sufficient amounts. Such catalysts would be doubly beneficial to the environment: First, they are produced by a clean solar method; second, they promise to be more effective than existing ones at reducing the damage caused by fossil fuels.
 
Additional applications for the solar-synthesized fullerene-like particles might stem from the fact that the eight-sided molybdenum disulfide molecules are metallic in character whereas the spherical ones are semiconductors. The intermediate particles are hybrids: A metal-like component is embedded within a semiconductor, a structure that could find new uses in the semiconductor industry – for example, in the manufacture of advanced sensors.
 
The study was conducted by Dr. Ana Albu-Yaron and Prof. Moshe Levy in the lab of Prof. Reshef Tenne of the Weizmann Institute’s Materials and Interfaces Department, together with Dr. Ronit Popovitz-Biro of the Institute’s Chemical Research Support, and by Prof. Daniel Feuermann and Dr. Eugene A. Katz in the lab of Prof. Jeffrey Gordon at Ben-Gurion University, in collaboration with researchers in Germany: Marc Weidenbach, Drs. Maya Bar-Sadan and Lothar Houben of the Forschungszentrum Jülich, and Dr. Andrey N. Enyashin and Prof. Gotthard Seifert of the Technische Universität Dresden.
 
Prof. Reshef Tenne is Head of the Helen and Martin Kimmel Center for Nanoscale Science; and his research is supported by the Phyllis and Joseph Gurwin Fund for Scientific Advancement. Prof. Tenne is the incumbent of the Drake Family Professorial Chair in Nanotechnology.


 
 
An intermediate molybdenum disulfide nanoparticle has an octahedral center and a spherical outer shape
Chemistry
English
  • Fullerene
  • Materials and Interfaces
  • Nanoparticles
  • Reshef Tenne

Nanotubes Go for Gold

English
 
 

Prof. Ernesto Joselevich and Tohar Yarden. Golden touch

 

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
King Midas may have turned everything he touched to gold, but he never got his hands on carbon nanotubes. That feat has been left to Prof. Ernesto Joselevich and research student Tohar Yarden of the Chemistry Faculty’s Materials and Interfaces Department. The two recently applied a technique, developed in Joselevich’s lab for directing the production of carbon nanotubes, to create, among other things, intricately shaped gold nanowires.
 
Two years ago, Joselevich and his research team began fabricating long, thin carbon nanotubes that bend, loop and curve into various shapes, including radiator-like serpentines that hinted at possible uses in nanodevices. Interestingly enough, these tiny shapes are self-organizing. By creating a sort of “ordered chaos,” in which fluctuations drive tube formation, the scientists found it was possible to “draw” any pattern they desired with these continuous carbon nanotubes.
 
 
Gold-plated serpentine carbon nanotubes

 

Now, the scientists have begun gold-plating these nanotube shapes through a process called electrodeposition. Electrical pulses cause dissolved metal salts to leave the solution and attach to the nanotubes. Lovely as these creations appear, the idea is not to design microscopic jewelry, says Joselevich, but “to combine the unique geometry of the serpentine nanotubes with the properties of other materials.” Any material that is a conductor or semiconductor can be used to make nanowires with this method, which, in their recent publication in Nano Letters, the researchers called "drawing with nanotubes.” In addition to gold, they coated serpentine nanotubes with bismuth telluride, a material that has the ability to convert heat to electricity or, conversely, cool when electricity passes through. Nanowires of bismuth telluride could power microscopic devices or function as miniature cooling units for nanoelectronics.
 
The team is continuing to experiment with creating serpentine nanowires from a variety of materials. A recent effort might be used, for instance, in miniature light collectors or sensors, or even “nano-neon” lights.
 
Prof. Ernesto Joselevich's research is supported by the Carolito Stiftung.
 



 

 

 
Gold-plated serpentine carbon nanotubes
Chemistry
English
  • Carbon nanotubes
  • Ernesto Joselevich
  • Materials and Interfaces
  • Nanoscience

Versatile Nanoparticles

English

“If you break a piece of gold in half,” says Dr. Dan Oron of the Institute’s Physics of Complex Systems Department, “Each of the halves will have the same basic properties as the original. But the same won’t be true if that piece of gold is just a few thousand atoms in size: Among other things, the color of the halves will be different.”
Dr. Dan Oron. Gold and crystal nanoparticles
 
 
We see color when light waves are absorbed or reflected from objects, and once the size of those objects gets down to the length of light waves or smaller, strange things begin to happen. In fact, many of the properties of familiar materials change dramatically at the nanometer scale (a billionth of a meter).
 
 
Oron has been investigating the characteristics of nanocrystals – seeking to understand the rules governing bits of matter around the size of protein molecules, and learning how to put those rules and properties to use. “Conventional inorganic chemistry gives us one kind of mechanism for creating materials – one with a limited set of ‘knobs.’ Nanoparticles could give us radically new machinery that could help us realize previously unattainable material properties.”

 
Just how versatile nanoparticles are can be seen in two of Oron’s recent research projects: In one, unique nanoparticles he is working on might be used to light up molecules under the microscope; in the other, they might power a new kind of solar collector.


Oron’s nanoparticles are semiconductors. When they’re disturbed – say by a photon (a light particle) striking them – an electron is briefly excited, leaving behind a positively charged hole. When the excited electron and the hole recombine, light is re-emitted. What happens when two electrons are excited in a single nanoparticle? Do the electrical charges occupying the nanocrystal interact, repelling or attracting each other? Oron has found that he can induce strong repulsion between the two holes by adding just a few atoms of a different element to the nanoparticles. This creates a cage that traps a single positive charge, repelling the second one as it does so and causing the light to be re-emitted from the particle in a different color.
 

In microscopes, such color-altering nanoparticles could be used as markers. Some of today’s advanced microscopes fire two photons at the same spot in rapid succession, causing the material to emit a brief flash of light. But light from tiny nanoparticles attached to cells or proteins could prove to be more stable and reliable. In addition, a nanoparticle could be designed to emit one color the first time it was struck by a photon and another color the second; or, to enhance resolution, it might scatter light only after being struck by two photons simultaneously. The challenge in creating such ultra-tiny markers, says Oron, lies on the one hand in controlling their fabrication to attain the desired structure and on the other hand in detecting extremely weak optical signals. The ability of nanoparticles to scatter light, for example, drops drastically as they shrink. These challenges, however, can be overcome. "We have recently succeeded in creating the smallest nonlinear light-scattering nanoparticle yet – less than 15 nanometers across. We managed to find a ‘sweet spot’ – one that works about ten times better than the equivalent bulk material,” he says.

 
Two-photon autofluorescence image of a live cell incubated with gold nanoparticles, superimposed on a simple transmission image of the cell
 
 
Oron’s solar collector research relies not on the nanoparticles’ light-emitting skills but rather on their ability to absorb light. The organic light absorbers in dye-based solar cells have a tough job: They must absorb quantities of light of a wide range of wavelengths, separate the electron from the “hole” it leaves, and then accept an electron back, over and over again. Oron and Dr. Arie Zaban of Bar-Ilan University knew that semiconductor nanoparticles could absorb sunlight much more easily almost across the entire visible spectrum but were much less efficient at splitting the charges. Many organic dyes, on the other hand, can separate charges reliably but are limited in the wavelengths of light they can take in, as well as in their long-term stability. Oron and Zaban had an idea: Why not divide the workload? The researchers created microscopic devices in which nanoparticles act as antennae, funneling the sun’s energy to dye molecules, where the charge separation takes place. Oron believes that with some tweaking, such combination solar collectors could be highly efficient.
 

Rehovot born and bred


Dr. Dan Oron was born in Rehovot in 1974 and grew up right around the corner from the Weizmann Institute. “I was at the Institute all the time and participated in many extracurricular activities here; my first was at age 11,” he recalls. He completed his B.Sc. and M.Sc. through the elite Talpiot army study program, the latter at Ben-Gurion University of the Negev, where he investigated the physics of turbulence. His doctoral research on spectroscopy using ultra-fast pulses of light was conducted at the Weizmann Institute in the group of Prof. Yaron Silberberg. Oron began working with nanoparticles during his postdoctoral research in the lab of Prof. Uri Banin at the Hebrew University of Jerusalem. He joined the Weizmann Institute in 2007.

Dan is married to Ruti; they have a daughter, Hila.
 
Dr. Dan Oron's research is supported by the Phyllis and Joseph Gurwin Fund for Scientific Advancement; the Wolfson Family Charitable Trust; Yossie Hollander, Israel; and Daniel S. Shapiro, UK.




 
 
Two-photon autofluorescence image of a live cell incubated with gold nanoparticles, superimposed on a simple transmission image of the cell
Chemistry
English
  • Dan Oron
  • Microscopy
  • Nanoparticles
  • Physics of Complex Systems
  • Solar Energy

How to Catch a Molecule

English
(l-r) Tamar Yelin, Roy Kazaz, Dr. Oren Tal, Regev Ben-Zvi and Ran Vardimon. Moving through molecules
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Can electronic components be the size of a single molecule? If such molecular components are incorporated into electronic devices, will they function like scaled-down versions of larger materials, or will they have unique properties that could be applied to create entirely new types of systems?

Dr. Oren Tal, who is currently setting up a new lab in the Chemical Physics Department of the Faculty of Chemistry believes that the best way to answer these questions is to look at the very basic issues _ how electrons move through molecules, for instance. “We’re interested in the basic principles – why it works and how it works. This kind of deeper understanding could lead to technological leaps in the future, and we might learn something new about our world in the process.”
 
To investigate molecules, Tal must first catch them. He releases the molecules inside a vacuum chamber whose interior is cooled to a chilly 4° above absolute zero. His molecule trap consists of a wire mounted on a piece of springy material. When the material is pushed up from below, the wire bends and breaks apart in a pre-weakened spot, opening up just enough for a single molecule to slip inside. With ultra-sensitive instruments that record the minuscule vibrations taking place in a single molecule, he can tell whether one has fallen into the trap and probe what happens to it when electrons pass through.
 
Different Disciplines, Same Subject
 
Dr. Oren Tal grew up on Moshav Ramot Hashavim, in central Israel. After completing an M.Sc. in chemistry at the Weizmann Institute, he received his Ph.D. from the Electronics Engineering Department of Tel Aviv University in the field of physical electronics. In his postdoctoral work, in the Physics Department of Leiden University, the Netherlands, Tal began capturing single molecules to investigate their electrical properties. “I studied in three different places, with three different disciplines: chemistry, engineering and physics; but my basic subject matter – conductivity in molecules – has been the same since the beginning,” he says.
 
Tal is married and the father of Jonathan, aged 3, and Lia, aged 1. His interests include Aikido and art.
In effect, the trapped molecule becomes part of an electronic circuit consisting of the molecule itself and the two pieces of the wire. How does the molecule affect the electrons’ flow? Can it change its properties or the properties of the circuit? Because the molecules in Tal’s experiments stick directly to the sides of the wire, he can pull on them, stretching the molecular bridges to see how this affects their conductivity. In his postdoctoral research, Tal started with the simplest molecules possible: hydrogen and water. He has now moved on to oligoacenes _ molecules in which benzene is a repeating unit. The most basic member in this family, benzene, is a ring containing six carbon atoms. The benzene molecule tends to slip into the trap disk-wise; but it gets tilted when pulled, so that it only touches the wire at two points on the ring. This movement changes the molecule’s conductivity, something like flipping a switch.

What makes a molecular bridge more or less conductive? That is, what determines how electrons pass through? Each molecular bridge restricts its electron flow to a limited number of channels. And because two electrons can’t be in the same state at the same time, if an electron is transmitted via a channel, a second electron cannot use that channel and will be rejected. Tal uses a method of identifying those channels by “listening for noise” – the signals from the electrons that bounce back from already-occupied channels.
 
Other research that Tal is planning to conduct in his Weizmann lab involves the new field of spintronics. This area is based on using electrons’ spin rather than the electrons’ charge that powers all conventional electronics. Electronic spin exists in one of two orientations, which are referred to as "up" and "down." Spintronic devices might be far more energy-efficient than today’s electronics and much faster, to boot. But scientists must first figure out how to manipulate and preserve electronic spin states in a controlled way. Tal intends to capture molecules with interesting shapes for these experiments: for example, molecules that twist into screw-like shapes. He thinks that movement in their spiral-shaped channels could favor the transmission of spins with a particular orientation.
 

Dr. Oren Tal's research is supported by the Carolito Stiftung and the estate of Lela London. Dr. Tal is the incumbent of the Alvin and Gertrude Levine Career Development Chair.

 

 
 
 
 
 
 

 

 
 

 
 
(l-r) Tamar Yelin, Roy Kazaz, Dr. Oren Tal, Regev Ben-Zvi and Ran Vardimon. Moving through molecules
Chemistry
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  • Chemical Physics
  • Electric current
  • Microelectronics
  • Nanoscience
  • Oren Tal
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Nanotube News

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Ronen Kreizman, Dr. Maya Bar Sadan, Profs. Daniel Wagner, Reshef Tenne and Ernesto Joselevich and Dr. Ifat Kaplan-Ashiri. Stacking, stretching and bending
 
 
 
 
 
 
 
 
 
 

 

 

Picture-Perfect Nanotubes

 
A chain is only as strong as its weakest link. This saying is true for materials as well; it is the defects that often ultimately determine a material's strength.
 
Nanotubes may buck this trend, as their defects are limited. Because of their infinitesimal size, however, it's hard to prove this experimentally.
 
The first synthesis of inorganic nanotubes, composed of tungsten disulfide, took place in the lab of Prof. Reshef Tenne of the Materials and Interfaces Department more than 15 years ago. When Dr. Ifat Kaplan-Ashiri, recently a student in Tenne's group in the Institute's Faculty of Chemistry, decided to investigate the mechanical properties of these nanotubes – made of compounds other than carbon – she turned to Prof. Daniel Wagner of the same department.
 
Wagner researches carbon nanotubes, and he has developed special techniques to probe their mechanical properties. Applying Wagner's techniques to multiwalled tungsten disulfide nanotubes synthesized in Tenne's lab by Dr. Rita Rosentsveig, Kaplan-Ashiri put them through a series of stretching, bending and compression "exercises" while observing their behavior under a scanning electron microscope. When her results were compared with theoretical values obtained by the group of Prof. Gotthard Seifert of Dresden University of Technology, Germany, the values matched almost exactly. In other words, the nanotubes were as strong as the theory predicted – virtually defect-free. Also participating in the project were Drs. Sidney R. Cohen and Konstantin Gartsman of the Chemical Research Support Department.
 

The Missing Link

 
Dr. Maya Bar Sadan, also a former Tenne student, researches the structural properties of nanotubes. Now a postdoctoral fellow at the Julich Research Centre, Germany, Bar Sadan uses state-of-the-art electron microscopy techniques to determine the structure of multiwalled, inorganic nanotubes, atom by atom.
 
A nanotube basically consists of a sheet of atoms that has been rolled up into a seamless cylinder. But that sheet can roll in different ways: "armchair-wise" on the horizontal plane, "zigzag" on the vertical plane or "chiral" diagonally. Multiwalled nanotubes are, like Russian dolls, composed of multiple cylinders nested inside one another.
 
Bar Sadan, working with Dr. Lothar Houben, found that the first two or three outer layers of multiwalled, inorganic nanotubes are always rolled identically, either armchair or zigzag. Inside is a chiral layer, with the innermost layers reverting back to either the armchair or zigzag conformation. Knowing the tubes' structure can aid in refining their growth mechanism, helping to create more perfect nanotubes with even greater strength.

 

Double Twist

 
Tenne also approached Prof. Ernesto Joselevich and his post-doctoral student Nagapriya Kavoori  also of the Materials and Interfaces Department, who had developed a method to test nanotube mechanics by twisting them.
 
Kavoori, together with Ohad Goldbart and Kaplan-Ashiri of Tenne's group, had a surprise: When twisted, the inorganic nanotubes started creaking like the hinges of an old door! This creaking – a type of friction known to physicists as "stick-slip" behavior – takes place in everything from earthquakes to violins, but it has never before been observed in twisting on the atomic scale.
 
What was causing this creaking? Preliminary observations showed that at the onset of twisting, the multiple walls "stick" and twist as one; but beyond a certain angle, the outer layer "slips" and twists around the inner walls.
 
Joselevich, Kavoori and Seifert came up with a simple theoretical model. Unlike smooth-walled carbon nanotubes, inorganic nanotubes have a bumpy, corrugated surface. As Bar Sadan's research had shown, the outer walls are rolled identically, so they stack up like sheets of corrugated tin. The scientists calculated that this stacking would cause the layers to stick initially; but when the force of the twisting became stronger than the "locking" force between the corrugated walls, it caused them to repeatedly stick and slip.

 
A Melting Pot

 
Another student in Tenne's group, Ronen Kreizman, working together with Oxford University student Sung You Hong under the supervision of Profs. Malcolm Green and Ben Davis, has discovered yet another way to get to the core of nanotubes – literally: By melting an inorganic material with a lower melting point than tungsten disulfide, Kreizman found that the liquid is drawn into a nanotube's straw-like hollow cavity, where it then solidifies into a new nanotube.
 
This is the first ever report of a perfectly crystalline inorganic nanotube being produced within a nanotube. Certain inorganic materials tend to be unstable in tubular form, resisting synthesis into nanotubes. Now, however, thanks to Kreizman, this feat seems to be possible.
 
Dr. Ronit Popovitz-Biro of Chemical Research Support and Dr. Ana Albu-Yaron of the Materials and Interfaces Department also participated in this research.  
 
multiwalled inorganic nanotube
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Prof. Ernesto Joselevich's research is supported by the Helen and Martin Kimmel Center for Nanoscale Science; the Gerhardt Schmidt Minerva Center on Supramolecular Architectures.
 
Prof. Reshef Tenne's research is supported by the Helen and Martin Kimmel Center for Nanoscale Science; and the Phyllis and Joseph Gurwin Fund for Scientific Advancement. Prof. Tenne is the incumbent of the Drake Family Professorial Chair in Nanotechnology.
 

Prof. Daniel Hanoch Wagner is the incumbent of the Livio Norzi Professorial Chair.

 
 

Ifat and Maya

 
Dr. Ifat Kaplan-Ashiri pursued her M.Sc. and Ph.D. degrees in the lab of Prof. Reshef Tenne, garnering many prizes and honors, the most recent being the Outstanding Ph.D. Student Award of 2007, bestowed by the Israeli Chemical Society. Kaplan-Ashiri recently started her postdoctoral studies in the group of Dr. Katherine Willets at the University of Texas at Austin where she intends to combine atomic force microscopy and Raman spectroscopy to study single molecules.
 
Israeli-born Kaplan-Ashiri is married to Elad; they have one daughter. Apart from nanotubes, she likes to research the "pleasurable properties" of playing the piano, pottery and reading.
 
Dr. Maya Bar Sadan received a B.Sc. in chemical engineering from the Technion and an M.Sc. from the Weizmann Institute, studying superconductors. Her Ph.D. research was carried out under Tenne. Investigating inorganic nanotube properties together with German colleagues, she found they can change from semiconductors to a metal-like state. Bar Sadan is a recent recipient of a National Postdoctoral Award for Advancing Women in Science.
 
The mother of an 8-year-old daughter and 6-year-old twins, Bar Sadan likes to hike and read in her spare time.
 
 
 
(l-r) Ronen Kreizman, Dr. Maya Bar Sadan, Profs. Daniel Wagner, Reshef Tenne and Ernesto Joselevich and Dr. Ifat Kaplan-Ashiri. Defect-free nanotubes
Chemistry
English
  • Chemistry
  • Daniel Wagner
  • Ernesto Joselevich
  • Materials and Interfaces
  • Nanotube
  • Reshef Tenne

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